1. 20-60 hours, depending on how many side quests you do, how much time you spend talking to your companions, how much time you spend experimenting with lightsaber crystals, etc.
2. I don't think so. However, the Jedi robes all come with cloaks.
3. There are large masses of enemies you can fight at certain points in the game(aka 10-15 people/animals at once).
4. Yes. There are Light options and Dark options everywhere. Reaching completely light and completely dark gives you bonuses. Also, some crystals are only usable by light or dark. Darkside makes your face and your companion's faces ugly. If you'd prefer to just have Sith eyes and not the wrinkled grey face, there is a mod for that. PM me for it or ask for it on the Mod Link Requests thread under Holowon Laboratories/Mod requests.